The present invention relates to a transistor circuit, and more particularly to a novel transistor circuit in which its current amplification factor can be controlled easily and freely.
The emitter-grounded current amplification factor h.sub.FE as one of parameters for evaluating the characteristics of a transistor is given by the following equation: ##EQU1## where .alpha. is the base-grounded current amplification factor. The factor .alpha.0 is given as follows: EQU .alpha. = .alpha. * .beta. .gamma. (2)
where .alpha. * is the collector amplification factor, .beta. is the base transport factor, and .gamma. is the emitter injection efficiency. A consideration will now be taken on the emitter injection efficiency .gamma. of an NPN-type transistor. The emitter injection efficiency .gamma. is given as follows: ##EQU2## WHERE J.sub.n is the current density according to electrons injected from emitter into base, and J.sub.p is the current density according to holes injected from base into emitter.
In this connection, J.sub.n and J.sub.p are respectively given as follows: ##EQU3## Therefore, the following equation is obtained. ##EQU4## where L.sub.n : diffusion length of minority carrier in base
L.sub.p : diffusion length of minority carrier in emitter PA1 D.sub.n : diffusion constant of minority carrier in base PA1 D.sub.p : diffusion constant of minority carrier in emitter PA1 n.sub.p : concentration of minority carrier in base at its equilibrium PA1 p.sub.n : concentration of minority carrier in emitter at its equilibrium PA1 V: voltage applied to emitter junction PA1 k: Boltzmann's constant PA1 I: temperature
If the impurity concentration of emitter is taken as N.sub.D and the impurity concentration of base is taken as N.sub.A, p.sub.n /n.sub.p can be replaced by N.sub.A /N.sub.D. Further, L.sub.n is limited by the base width W and L.sub.n = W can be satisfied, so that the following equation is given: ##EQU5## The diffusion constants D.sub.n and D.sub.p are the function of the carrier mobility and temperature and regarded as constant.
As will be apparent from the aforesaid equation, in order to enhance h.sub.FE of a transistor, it is enough to reduce the value of .delta..
Thus, in normal transistors, the emitter impurity concentration N.sub.D is made quite large in order to reduce the value of .delta..
However, if the emitter impurity concentration N.sub.D is made quite large such, for example, as more than about 10.sup.19 atoms/cm.sup.3, the lattice defect, dislocation and the like occur and hence the perfectness of a crystal can not be attained. In addition, since the emitter impurity concentration is high, the lifetime .tau..sub.p of minority carriers injected thereinto from the base is shortened, and the diffusion length L.sub.p of these minority carriers or holes becomes small according to the following equation: EQU L.sub.p = .sqroot. D.sub.p .tau..sub.p ( 8)
Accordingly, as is obvious from the equation (7), the value of .delta. can not be made so small and the injection efficiency .gamma. can not be enhanced more than a certain extent. As a result, the value of h.sub.FE can not be enhanced so much.
A special transistor free from such a defect is hereinafter described. This special transistor can be considered as on NPN-type or as a PNP-type, but an NPN-type transistor will be now described, by way of example, with reference to FIGS. 1 and 2.
As shown in FIG. 1, a semiconductor substrate S is provided having a first semiconductor region 1 of N type, a second semiconductor region 2 of P type disposed adjacent to the first region 1, and a third semiconductor region 3 of N.sup..sup.- type disclosed adjacent to the second region 2. A first PN junction J.sub.E is formed between the first and second regions 1 and 2, and a second PN junction J.sub.C is formed between the second and third regions 2 and 3, respectively.
A potential barrier is formed within the first region 1 opposing to the first junction J.sub.E at a position spaced from the junction J.sub.E by a distance smaller than the diffusion length L.sub.p of minority carriers or holes injected into the first region 1 from the second region 2. In the illustrated embodiment, the impurity concentration of the first region 1 is made quite low such as on the order of 10.sup.15 atoms/cm.sup.3, and also an N type region 1a having high impurity concentration on the order of 10.sup.20 atoms/cm.sup.3 is formed in the first region 1 to form an L-H junction J.sub.H in the region 1, thus causing a potential barrier to be formed therein.
The impurity concentration of the second region 2 is selected to be on the order of 10.sup.15 to 10.sup. 18 atoms/cm.sup.3, and that of the third region 3 is selected to be quite low, such as on the order of 10.sup.15 atoms/cm.sup.3.
Further, in the third region 3, there is formed a high impurity concentration region 3a of the same conductivity type which is spaced from the second junction J.sub.c. The concentration of this region 3a is selected to be on the order of 10.sup.19 atoms/cm.sup.3.
A first electrode 4E is deposited on the high impurity concentration region 1a of the first region, in ohmic contact therewith, and similarly a second electrode 4B and a third electrode 4C are deposited on the second region 2 and on the high impurity concentration region 3a of the third region 3, respectively, in ohmic contact therewith. First, second and third terminals E, B and C are respectively led out from the electrodes 4E, 4B and 4C. Further, reference numeral 5 indicates an insulating layer, such as SiO.sub.2, formed on the surface of the substrate S.
The above-mentioned element is used as a transistor. In this case, the first, second and third regions 1, 2 and 3 serve as emitter, base and collector regions, respectively, and the emitter junction J.sub.E is applied with a forward bias voltage while the collector junction J.sub.C is applied with a back bias voltage.
With such an arrangement, a hole injected from the base region 2 (the second region) into the emitter region 1 (the first region) is caused to have a long lifetime due to the low impurity concentration of the emitter region 1, superior crystalline property and the like, and hence the diffusion length L.sub.p of the holes in the emitter region 1 becomes long. However, even though the diffusion length L.sub.p is made long, if the injected holes reach the surface of the substrate S and are subjected to surface recombination, in a practical case, the diffusion length L.sub.p can not be made substantially long. With the above described construction, however, since the potential barrier is formed opposing the emitter junction J.sub.E at a distance therefrom smaller than the diffusion length L.sub.p, the surface recombination is decreased and the diffusion length can be regarded as sufficiently long.
Thus, there is an effect that the current component J.sub.p of the holes injected into the emitter region 1 from the base region 2 is reduced by the provision of the potential barrier. That is, in the emitter region 1 there occurs the difference of quasi Fermi levels or the built-in field at its L-H junction which acts against the diffusion of minority carriers or holes. Therefore, when the level is sufficiently high, the diffusion current caused by the concentration gradient of the holes and the drift current caused by the built-in-field are cancelled out at the L-H junction J.sub.H to reduce the hole current J.sub.p injected from the base through the emitter region 1 of low impurity concentration. Of the current component passing through the emitter junction J.sub.E the ratio of the electron current reaching the collector region 3 is enhanced due to the above effect. Thus, as is apparent from the equation (3), the value of the emitter injection efficiency .gamma. becomes large and h.sub.FE is enhanced.
This level difference (the height of the potential barrier) is desired to be larger than 0.1 eV. The value of the built-in-field at the potential barrier is required to be larger than kT/qL.sub. p and in particular is desired to be more than 10.sup.3 V/cm. In the case when the L-H junction J.sub.H is formed as illustrated, the potential barrier of 0.2 eV can be formed by properly establishing the impurity amount and the gradient of the high impurity concentration region 1a.
In an example of FIG. 2, the high impurity concentration region 1a is provided in the first region 1 to form the potential barrier, and also a P-type additional region 6 is provided in the first region 1 to form a PN-junction J.sub.S in opposition to the first junction J.sub.E. Also in this case, the distance between the PN-junction J.sub.S and the junction J.sub.E is selected shorter than the diffusion length L.sub.p of the minority carrier in the first region 1.
With the above-mentioned construction, the holes injected into the first region 1 effectively reach the additional region 6 because of their long diffusion length as mentioned above and are absorbed into the P-type additional region 6. When the additional region 6 is electrically isolated, its potential is raised according to the increase of the holes to forwardly bias the PN-junction J.sub.S formed between the region 6 and the first region 1 up to substantially its rising-up voltage and the holes are reinjected into the first region 1. For this reason, the concentration of holes in the first region 1 near the additional region 6 is enhanced. Accordingly, the concentration distribution of holes between the junctions J.sub.E and J.sub.S of the first region 1 is uniformed to make its gradient gentle with the result that the diffusion current J.sub.p flowing from the second region 2 to the first region 1 is decreased.